For ease of exposition and understanding, the descriptions and examples presented herein relate to optical imaging of the retinal nerve fiber layer (RNFL). For those versed in the art it will be clear that the methods described apply equally well in other applications. In particular, the applications to RNFL thickness and orientation applies to other materials exhibiting orientation dependent form birefringence.
The retinal nerve fiber layer (RNFL) is formed by highly ordered optic nerve fiber bundles comprised of parallel axons containing small diameter cylindrical cell structures. The diameters of the cylindrical cell structures are smaller than the wavelength of light. These nerve cells transmit the visual signal generated by the photoreceptors along the optic nerve to the brain.
In glaucoma and certain other diseases, these nerve fibers can become damaged. Glaucoma is a term used to describe a group of diseases characterized by the loss of retinal ganglion cells and their axons and is one of the leading causes of blindness in the world. In many cases, vision loss due to glaucoma is irreversible. Glaucoma diagnosis is most commonly associated with an increase in intraocular pressure (IOP); however, diagnosis may also be based on the assessment of the optic nerve head (ONH), visual function, and/or the health and thickness of the RNFL. RNFL defects are early signs of glaucoma and frequently occur prior to measurable visual field loss. Since retinal function damage, at present, is irreversible, early detection and treatment are highly sought out for improved outcomes. The demonstrated high correlation between RNFL thickness and retinal function allows for early detection of potential retinal damage by means of retinal thickness measurements.
In recent years, scanning laser polarimetry (SLP) has demonstrated highly reliable indirect measurement of RNFL thickness. SLP uses a confocal scanning laser ophthalmoscope to measure retardation of the optical signal through the RNFL. The optical retardation is converted into thickness by means of a correlation that has been validated by histology. Under SLP imaging, the highly ordered, parallel fiber bundles of the RNFL show form-birefringence with the slow axis parallel to the direction of the fibers.
Another optical imaging technology is Optical Coherence Tomography (OCT). OCT has sufficient imaging resolution to measure the RNFL directly. Polarization sensitive (PS)-OCT can be used to also detect the optic axis of a sample. The optic axis can be determined using multiple interrogations of an RNFL region (de Boer, J. F. et al., Optics Letters 24: 300-302, Roth, J. E. et al., Optics Letters 26, 1069-1071) or the fast optic axis can be determined by use of a single A-scan (Hitzenberger, C. K. et al., Optics Express, 13, 780-790). In either case, PS-OCT can be used to determine fiber orientation within the RNFL.
Current SLP and OCT measures of NFL thickness are improving early detection of decreased eye function due to reductions in nerve tissue capacity. Still, there is a need for increased specificity and sensitivity of medical diagnosis from ophthalmological measurements.